The Moons of Venus

Moons of Venus

In 1672, Italian-French astronomer Giovanni Cassini spotted a small object close to Venus, which he identified as a possible moon. Although others thought they spotted the satellite, which was named Neith, astronomers eventually concluded that sightings were only stars. There were no moons of Venus.

This is Francesco Fontana's drawing of the supposed satellite(s) of Venus. Woodcuts from Fontana's work in 1646. Note the fringes of light around Venus, which are produced by optical effects.

Thus, Venus joined Mercury as the only planets in our Solar System without satellites. The question then became, "Why?" Moons are certainly plentiful. At least 169 satellites orbit the eight planets, with another four worlds circling tiny Pluto. Even some asteroids have moons. Did these two inner worlds ever have moons? And if so, where did they go?

One theory for why Mercury and Venus lack moons involves the strong tidal pull from the Sun. It might simply be impossible for these worlds to hold onto to any satellites for very long. This would particularly affect Mercury, which is a small world closest to the Sun. Venus, however, is further away and roughly the same size of Earth. Why couldn't it hold onto a moon?

To find out, let's first look at the three ways that moons are formed. They can be created from debris left over from when their planets formed. A world can capture a wandering asteroid into its orbit, which is what Mars might have done with Phobos and Deimos. Or a satellite can result from a massive collision between a planet and another body, which is believed to be the way Earth's Moon was formed.

In 2005, two Caltech researchers, Alex Alemi and David Stevenson, proposed that Venus had not one but two moons in its early history, both caused by collisions with other bodies. The planet then lost both satellites for different reasons involving gravitation pull.

No world was immune to the enormous number of collisions that occurred in the early Solar System. Alemi and Stevenson believe believe that Venus was hit early on by a very large object, resulting in a new moon. However, that world slowly spiraled away from the planet due to the Sun's strong tidal forces. This effect has been documented with other planetary systems. Both Earth's moon and Mars' Deimos are slowly moving away from their planets.

Alemi and Stevenson believe that about 10 million years after the first collision, Venus was hit again by another large body. The force of the collision was so strong that it knocked the planet into a retrograde spin that is the opposite of how other worlds rotate. A new moon was created, but because of the energy required to reverse Venus' rotation, the satellite had an unstable orbit. It eventually spiraled in and collided with Venus.

Something very similar is happening at Mars, where Phobos is slowly moving toward the planet. Millions of years from now, the tiny moon will break up into a ring or impact on the surface.

Absent a time machine, we will never know for sure what happened to the moons of Venus. It will remain one of those mysteries of the Universe that can never be definitely explained.